HR: 11:50h
AN: V42A-06    [Abstracts]
TI: What does hotspot-ridge interaction tell us about mantle plumes?
AU: * Murton, B J
EM: bjm@soc.soton.ac.uk
AF: Southampton Oceanography Centre, European Way, Empress Dock, Southampton, SO14 3ZH United Kingdom
AB: While the scientific community is currently contending the origin and nature of 'plumes', there is no doubt that close proximity to hotspots affects the structural, volcanic and geochemical character of spreading ridges. We can consider two end-member processes that may account for the 'plume' phenomenon: the classic and current paradigm of a column of rising mantle, made buoyant by an excess of temperature and originating deep within the mantle, or a non-dynamic mechanism involving a geochemical anomaly, residing passively in the shallow mantle, that melts spontaneously as a result of intraplate stresses or asthenospheric motion. While a priori, both mechanisms may hold true, they can be distinguished for individual 'plumes' where they interact with a spreading ridge that leaves a lithospheric and crustal trail recording the history of that interaction. The Reykjanes Ridge, southwest of Iceland, has all the attributes associated with a ridge-centred 'plume'. These include: thickening of the crust, shallowing of the ridge axis, an increase in segment length, and enrichment in geochemical tracers of the 'plume' mantle. Evidence for dynamic interaction between ridge and plume comes from southward closing V-shaped ridges, centred on the plate boundary, that indicate southward advection of plume mantle away from Iceland. Geochemical tracers include incompatible trace element enrichment and isotope ratios (e.g. Sr, Nd, Pb and He) that show multi-component mixing between several depleted and enriched sources. These sources are resident in the mantle beneath the Reykjanes Ridge and show modification by melting processes, consistent with a history of advection away from Iceland. Further evidence for interaction between a dynamic 'plume' and spreading ridge comes from the R‚union-Central Indian Ridge couplet, which comprises an off-axis plume and medium-rate spreading centre. Here, the ridge also exhibits attributes associated with 'plume' influence such as shallowing depth, increase in segment length, and multi-component geochemical mixing that varies with distance along the plate boundary. Both the Icelandic and R‚union 'plume'-ridge couplets provide compelling evidence for advection and lateral flow of 'plume' mantle away from a central source and subsequent mingling with depleted, sub-ridge mantle characteristic of MORB genesis. As such, these studies support the current paradigm for mantle 'plumes' as phenomena associated with actively upwelling material that has resided deep within the mantle and whose influence upon spreading ridges is profound.
DE: 1749 Volcanology, geochemistry, and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly